This invention relates to a method and apparatus for casting a plurality of ingots. More particularly, this invention relates to a method and apparatus for casting a plurality of ingots from one or more electrodes and for eliminating the formation of deep molten cores in the ingots.
For a long time, the manufacture of ingots according to any one of several consumable electrode processes was generally limited to melting or remelting an electrode into an ingot whose cross-sectional area was larger than that of the electrode. Typically, the maximum ratio of electrode cross-sectional area to ingot cross-sectional area ranged up to about 80 percent. Although it was desirable to manufacture an ingot having a cross-sectional area smaller than the cross-sectional area of the electrode, metallurgical and economic restrictions made this difficult to achieve.
Recently, a method and apparatus was invented for casting a plurality of ingots which allowed the rates of the electrode cross-sectional area to the ingot cross-sectional area to exceed 100 percent. That invention is set forth in U.S. Pat. No. 3,782,445 for Method of Casting a Plurality of Ingots in a Consumable Electrode Furnace, issued to Luchok et al. on Jan. 1, 1974. The invention described therein permits casting a plurality of ingots while avoiding the aforementioned metallurgical and economic restrictions.
The cost of ingot manufacture by electroslag melting or other processes is in part a function of the rate of solidification of the ingot. In addition, the metallurgical quality of the ingot is a function of the solidification rate. The solidification rate, in turn, is a function of the ingot cross-sectional area. The smaller the ingot cross-sectional area, the lower the permissible solidification rate and, consequently, the smaller the number of pounds of ingot which can be cast per minute. Stated otherwise, the tolerable solidification rate for a given alloy is determined by the metallurgical characteristics and quality standards established by specification and, as a general rule, the tolerable solidification rate decreases with decreasing ingot cross-sectional area. Low ingot solidification rates, however, result in poor furnace utilization, increasing the unit cost of producing ingots.
Furthermore, the cost of manufacturing the electrode used in a consumable electrode process increases as the electrode diameter decreases. Consequently, prior to the invention set forth in U.S. Pat. No. 3,782,445, the manufacture of small ingots by consumable electrode processes and, in particular, the electroslag remelting process, had not been widely adopted for use in the manufacture of ingots of small cross-sectional area.
In U.S. Pat. No. 3,782,445, an apparatus and process is described for manufacturing small ingots by consumable electrode processes using electrodes of relatively wide diameters. In one application of the invention described therein, a molten metal pool is formed by melting an electrode according to classic electroslag remelting principles. The process entails suspending an electrode so that its lower end is immersed in a liquid slag bath. The lower end of the electrode is melted by passing current through the electrode and slag so that molten metal droplets form on the electrode and drop through the slag layer to form a molten metal bath. A plurality of ingots may be withdrawn from the molten metal bath.
Although the above method and apparatus permits the economical casting of a plurality of ingots, during casting an ingot may develop a deep molten metal core. The deep molten metal core causes a decrease in the rate of solidification of the ingot and, therefore, is undesirable. It is believed that the reason for the formation of the deep molten metal core is the attractive forces generated by the electrical current flowing in adjacent ingot mold sections.
It is well-known that an attractive force is generated between adjacent conductors which carry current in the same direction. For adjacent ingot sections, it is believed that the flow of electrical current in each ingot mold section creates an attractive force, and the force causes molten metal to flow out of an ingot mold section and back into the molten metal pool in the main mold section. As a result, the molten metal is circulated in the ingot mold section, and the circulation of the molten metal interferes with the gradual advance of the solidification front, i.e., the interface between the molten metal and the solidified ingot in the ingot mold section. The solidification front, then, remains relatively deep within the ingot mold section, and the ingot being cast has a deep molten metal core.
A principal advantage of the present invention is that a plurality of ingots may be cast from a pool of molten metal without the formation of deep molten metal cores in the ingots.
Another advantage of the present invention is that a plurality of ingots of satisfactory metallurgical quality can be cast at increased solidification rates, resulting in more efficient furnace utilization and decreased production costs.
Other advantages will be apparent from the description of the invention appearing below.